Apple has backed Microsoft in its battle to throw out a US warrant ordering it to produce emails stored in Ireland.

The software giant was ordered to hand over emails held in its Irish data centre by a US magistrates judge last December, as part of a criminal investigation.

But in an appeal last week the firm likened the warrant to a court authorising US agents "to break down the doors" of its Dublin facility.

Apple has now teamed up with network company Cisco to file a 'friend of the court' brief backing Microsoft's federal court challenge.

They said the US government should rely on mutual legal assistance treaties between countries to get such information, rather than "placing the burden" on technology companies.

The brief said: "The magistrate's analysis places providers and their employees at significant risk of foreign sanctions, and threatens a potential loss of customer confidence in US providers generally.

"It also encourages foreign law enforcement to take reciprocal actions by using equivalent foreign laws to require production of data stored in the United States, despite disclosure prohibitions in US law."

In April the judge that served the warrant rejected Microsoft's first attempt to quash the order because the 'search' itself would actually occur in the US.

However the Electronic Frontier Foundation says that as the seizure occurs in Ireland the US' Fourth Amendment - protecting against unreasonable search and seizure - should apply.